This invention generally relates to insulators for covering a high voltage component having an irregular surface, and is specifically concerned with an insulating wall structure for covering a high voltage coil having Roebeled strands on its outer surface that tend to create unwanted void spaces in the insulating material forming the wall.
Wall structures for insulating the high voltage coils used in power-producing alternators are known in the prior art. Such wall structures are made after the coil strands have been consolidated into a coil stack by applying strips of a material treated with an insulative, thermosetting epoxy over the top and bottom surfaces of the coil stack. The coil and the strips of epoxy-treated material are then placed within a heatable press assembly which simultaneously heats and compresses the strips of epoxy-treated material which causes them first to liquefy and to fill the space between the irregular surfaces on the coil exterior, and then to harden. A mica tape ground wall is formed around the consolidated coil and vacuum impregnated with an epoxy resin. The ground wall is completed by applying a conductive varnish over the impregnated mica tape. The resulting insulated coil is then assembled within a power-generating alternator.
In order to minimize the losses which would occur in these coils as a result of unwanted eddy currents, the windings of these coils are intertwined in a braid-like form known as a Roebel bar. The outer surfaces of such Roebel bars are highly discontinuous, as the relatively thick and flat strands do not smoothly align with one another on the same plane on the surface of the coil, but instead disjointedly overlap. The surface discontinuities presented by the twisted and overlapping Roebeled strands tends to create a number of air gaps in the outer surface of the coil (known as "void spaces" in the art) which provide potential sites for unwanted arcing if each such gap is not completely filled with insulating material.
Unfortunately, the prior art technique of applying hardenable, melted epoxy material under pressure to the outer surface of such coils does not always completely succeed in filling all of the arc-inducing voids around the outer surface of the high voltage coils. Consequently, in many of these coils, air bubbles remain in the epoxy insulating material after it hardens under pressure. These air bubbles can form sites of electric discharges when the coils are operated at their normal voltages of around 20 kilovolts, and experience has shown that even small amounts of arcing in a relatively few number of void spaces can severely curtail the expected 40 year life span of such coils to less than 20 years.
Clearly, there is a need for a new technique of applying an insulating wall around high voltage coils having Roebeled strands on their outer surfaces which is more effective in eliminating the presence of any void spaces between the windings of these coils. Ideally, such a wall structure should be capable of not only reducing the number of such spaces, but of further reducing the electrical stresses which occur within these voids. Finally, it would be desirable if the fabrication of such a new and improved wall structure were compatible with previous manufacturing devices and techniques, so that the manufacturing facilities for such structures would not have to be completely replaced.